1. Field of the Invention
The invention in general relates to magnetometer signal processing apparatus and particularly to apparatus for obtaining the magnetic signature of a marine vessel.
2. Description of the Prior Art
During the construction of a steel ship, many parts become magnetized and retain the magnetization. Other parts become magnetized by induction in the earth's magnetic field. The ship then acquires a magnetization which causes deviation of the magnetic compass and which may trigger magnetic mines or other explosive devices when the ship passes near them.
Neutralization of the strength of the magnetic field of the ship is accomplished by means of suitably arranged electric coils permanently installed at various locations in the ship. These coils are known as degaussing coils and the current provided to the degaussing coils is adjusted so as to try to maintain magnetic field neutrality. Thus, the ship will appear "invisible" to a magnetic mine.
In order to have some idea of what currents to provide the degaussing coils, the "magnetic signature" of the ship must be obtained. One method of obtaining this signature, or ship's magnetic variation, is to relatively pass the ship over an array of magnetometer devices, the output signals of which are affected by the passage of the ship past the array. A signal processing circuit is generally provided for processing a respective magnetometer output signal and in one widely used system, the magnetometer is of the type which provides an output signal indicative of only a change in the earth's magnetic field at the location, as opposed to the absolute value of it. This is accomplished by means of a compensating coil which is provided with current to counteract the earth's magnetic field so as to null the instrument.
The signal processing apparatus generally includes a meter for each respective magnetometer signal processing circuit, in addition to a recording means for making a permanent record of the magnetic deviations.
The degree of magnetization of the ship, and accordingly, the magnitude of the signal in each signal processing channel, is a function of the constructional materials utilized in the ship. For example, a wooden or aluminum ship might produce a minute change; whereas a completely steel or iron ship might send the readout instruments off-scale. Accordingly, to accommodate for these variations, the signal processing channels include means for varying the gains thereof, and a typical system might have ten or more gain ranges so that, for example, the maximum deviation caused by a wooden vessel will be a full scale movement, with one gain range setting, as would a steel ship with another gain range setting.
To set these various gain ranges, the signal processing channels include a plurality of sets of resistors having different values in accordance with the desired gain. The apparatus requires a relatively large amount of space to accommodate all of the necessary resistor sets, and in addition, a plurality of motor-driven rotary switches is required so that an operator may change the gain range without the requirement for manual intervention for all of the sets of resistors. In addition to the space requirement for these motor-driven switches, there is an additional power requirement and the objection of mechanical wear.